Look at yourself at the mirror, twice. The second time, every possible part of you has changed in you: your atoms, your energy states, your mind, etc.
The second time you look yourself at the mirror, you are looking at a second instance of you, which is clearly, overwhelmingly different from that of the first look (same happens with any other entity, rocks, houses, rainbows, etc.). There is the answer: strictly, concrete objects never exist as a single physical instance, change is permanent. While a concrete object seems static, it is never the same.
Where's the issue here?
In thermodynamics, there's a contrast in perceptions: microstatically, all systems change permanently (e.g. internal particles exchange energy constantly), while macrostatically, things seem static (constant temperature of a body is possible). Even ideas are subject to such principle (not in thermodynamics, evidently): two different thoughts have different parts. However, they can refer to the same concept.
So, the problem is not the possibility of multiple instances of concrete objects, but unique instances of abstract objects: how are immutable concepts / types possible? How is mathematics possible? That should be the question. No answer here, but the Kant's Transcendental Aesthetics section of his Critique of Pure Reason provides some insights worth considering.
Technical remark: an abstract object requires of two perceptions (two different instants of time) to be compared and found to be the same.